As environmental regulations of automobile markets are increasingly tightened, the world's carmakers have tried to develop environment-friendly and high-efficient cars. It is expected that the fuel-cell car will emerge as the center focus of an automobile market in the future because it is environmentally friendly in that it only discharges water while having high efficiency. As a result, many carmakers have continued to develop a fuel-cell car. A demand for a fuel-cell technology has continuously increased in the industrial world and the academic world. Therefore, research into a fuel cell for a car, for example, a proton exchange membrane fuel cell (PEMFC), has been actively conducted.
An operating temperature of a general PEMFC shown in FIG. 1 is less than 100° C. This occurs when water is generated due to the reaction of hydrogen with oxygen, which is a liquid phase. When the liquid phase water is discharged while flowing in a fuel cell channel along with air, this is called a flooding phenomenon. The flooding phenomenon has been known as one of the main factors that degrade PEMFC performance. However, since it is difficult to visualize a two-phase flow in the PEMFC channel, research into the flooding has been mainly conducted through interpretation using a CFD.
In addition to the CFD, a method of observing the flooding by visualizing the two-phase flow in the PEMFC channel has been used. As an example, there is a method of observing the flooding in the PEMFC channel by using a neutron beam. Even though there is no need to change the components of the PEMFC in order to observe the behavior of water using the neutron beam, and reliability is at the highest, an apparatus generating the neutron beam is very expensive. In addition, it is difficult to specifically know how water is discharged since processing the image data is complex and the interface of water with air is unclear. In addition, since the image using the neutron beam is implemented in a two-dimensional plane, it is difficult to know whether or not water is presented in an anode or a cathode state.
Therefore, researchers in and outside the country have mainly used a visualization method using a transparent window. The method can directly visualize the two-phase flow within the channel, such that it can specifically observe the shape where water flows. However, unlike the real PEMFC, since a visualization apparatus using the visualization method uses a current collector plate 600 in which a passage is formed on a metal plate having a thickness of about 1 mm as shown in FIG. 2, instead of using a separator 60 shown in FIG. 1 in order to perform visualization, it has a larger possibility of distorting the two-phase flow within a channel 610, as compared to the real PEMFC. Therefore, it is very important to secure the reliability by comparing an I-V characteristic curve between the real PEMFC and the visualization apparatus.
Generally, as shown in FIGS. 2 and 3, the visualization apparatus includes transparent plates 700 at the outer sides of the current collector plates 600 and large visualization windows 810 and 820 fixed on fixing frames 800 by being pressurized to the fixing frames 800 at the outermost sides of the visualization apparatus. The visualization apparatus observes the channels 610 of the current collector plates 600 through the large visualization windows. However, the result of the experiment show that in the real PEMFC using graphite as shown in FIG. 4, a maximum current density is 0.71 W/cm2, but in the visualization apparatus, a maximum current density is only 0.55 W/cm2 under the same conditions. In particular, in the fuel cell, the maximum current density is 2.02 A/cm2 but in the visualization apparatus, the maximum current density does not exceed 1.2 A/cm2. This implies that the flow phenomenon of the visualization apparatus is different from the phenomenon shown in the real PEMFC to cause the problem in reliability.